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Large-hydrocarbon fuels are used for ground and air transportation because of their
high energy-density and will be for the foreseeable future. However, combustion of
large-hydrocarbon fuels in a turbulent environment is poorly understood and difficult
to predict. The turbulent flame speed, which is the velocity at which a flame front
propagates through a turbulent fuel and air mixture, is a key property in turbulent
combustion. The turbulent flame speed can be used as a model input parameter for tur-
bulent combustion simulations. However, turbulent flame speeds for large-hydrocarbon
fuels are largely unknown. These values are needed to improve combustion models and
enhance understanding of the physics and chemistry that control turbulent combustion
of large-hydrocarbon fuels.
The objective of this study is to measure the turbulent flame speed of large-hydrocarbon
fuels and to identify key physics in the turbulent combustion of these fuels. This is
motivated by the use of the turbulent flame speeds in modeling combustion in practical
devices and the significant use of large-hydrocarbons in these devices. This research has
broad implications for society and industry; both the Federal Aviation Administration
and gas turbine engine companies have called for research on the turbulent flame speeds
of large-hydrocarbon fuels.
The turbulent flame speed in this work is defined as the global consumption speed,
and is measured for three fuels on a turbulent Bunsen burner. The Reynolds number,
turbulence intensity, preheat temperature, and equivalent ratio can be independently
controlled using the burner. A conventional Jet-A fuel, known as A2, is used as a
reference because of its common use in commercial and military aviation. A2 is compared
to bi-modal and quadra-modal blends referred to as C1 and C5, respectively. These fuels
are selected as they have similar heat releases and laminar flame speeds. Time-averaged
line of site images of OH*, CH*, and CO₂* chemiluminescence are used to determine an
the average flame front area. This flame area is used to determine the global consumption
speed. The global consumption speed is measured for Reynolds number and equivalence
ratio ranging between 5.000-10.000 and 0.7-1, respectively. Turbulence intensities are
varied between 10% and 20% of the bulk flow velocity.
The global consumption speed increases with turbulence intensity and Reynolds number
for all fuels. Global consumption speeds for A2 and C5 match within 5% at all conditions.
Conversely, the global consumption speed of C1 is up to 22% lower than A2 or C5.
These results indicate the global consumption speed is sensitive to turbulent velocity
fluctuations, bulk flow velocity, and fuel chemistry. These results together suggest the
global consumption speed is additionally sensitive to flame stretch.
Dimensional analysis is used to isolate and identify sensitivities of the global consumption
speed to turbulent velocity fluctuations, bulk flow velocity, global stretch rate, and fuel
chemistry. A clear sensitivity to fuel chemistry is observed and is affected by aromatic
and alkane content. A2 and C5 have higher global consumptions speeds and increased
stability; these fuels have shorter average hydrocarbon chain lengths and higher aro-
matic content than C1. In addition, the global consumption speed is highly sensitive to
turbulence intensity of the flow; the turbulent flame speed increases an average of 30%
for all fuels between the minimum and maximum turbulence intensity cases. Results are
attributed to a strong sensitivity of the global consumption speed to flame stretch and
a strong coupling of turbulence and fuel chemistry effects. These conclusions agree with
the available literature and provide a foundational understanding of the sensitivities of
the global consumption speed for large-hydrocarbon fuels.